Metal oxide varistor with laterally spaced electrodes

ABSTRACT

A metal oxide varistor having an alpha in excess of 10 in the current density range of from 10 3 to 102 amperes per square centimeter is formed with laterally spaced electrodes adjacent a first surface. A spaced third electrode may be associated with the first surface or a second surface. To improve the current carrying capacity of the varistor body the conduction gap between the electrodes may extend along the first surface an extended distance greater than the width of the surface. The conduction gap width may be varied continuously or in discrete steps.

United States Patent [1 1 Harnden, Jr.

[ METAL OXIDE VARISTOR WITH LATERALLY SPACED ELECTRODES [75] Inventor:John D. l-larnden, Jr., Schenectady,

[73] Assignee: General Electric Company,

Syracuse, N.Y.

221 Filed: July 22,1971

[21] App1.No.:165,0011

[52] US. Cl 338/20, 29/610, 29/613 [51] Int. Cl ..H01c 7/10 [58] Fieldof Search 338/13, 20, 21'

[56] References Cited UNITED STATES PATENTS 3,271,591 9/1966 Ovshinsky317/234 V Oct. 23, 1973 5/1959 Dalton 338/20 X Primary Examiner-C. L.Albritton Attorney-Robert J. Mooney et al.

[57] ABSTRACT 13 Claims, 13 Drawing Figures rAltminncias ms 3.768.058SHEET 2 BF 2 Q9 F|G.ll. I S 1 i L Y3- Y4 i i Y5- vs T T INVENTOR:

HIS ATTORNEY.

D. HABiDENJR.

METAL OXIDE VARISTOR WITH LATERALLY SPACED ELECTRODES My invention isdirected to a circuit component including a metal oxide varistor havinglaterally spaced electrodes.

It can be generally stated that the current which flows between twospaced points is directly related to the potential difference betweenthe points. For most known substances current conduction therethrough isequal to the applied potential difference divided by a constant, whichhas been defined by Ohms law to be its resistance. There are, however, afew known substances which have been observed to exhibitnon-linearresistances and which require resort to the following equation (1) torelate quantitatively current and voltage:

where V is the voltage between two points separated by a body of thesubstance under consideration, I is the current flowing between the twopoints, C is a constant, and alpha is an exponent greater than 1. Thereare many known electrical circuits in which it is quite desirable toincorporate one or more functional elements having non-linear orexponential resistance characteristics. For example, the non-linearresistance properties of silicon carbide have been widely utilized incommercial silicon carbide varistor's. Typically'silicon carbidevaristors exhibit an alpha of no more than 6.

It has been recently appreciated that varistors having alphas in excessof 10 within the current density range of 10 to 10 amperes per squarecentimeter may be made from bodies which are comprised of metal oxides.The metal oxide body may be formed predominantly of zinc oxide withsmall quantities of one or more other metal oxides being present. Metaloxide varistors having alphas in excess of 10 are disclosed in CanadianPat. No. 831,691, issued Jan. 6, 1970, for example. While the alphas ofthese metal oxide varistors are identified by the current density rangeof 10" to 10 amperes per square centimeter, which characteristicallyexhibits substantially constant alphas, it is appreciated that theiralphas remain high also at higher and lower currents, although somedecline from maximum alpha values have been observed.

The construction of a conventional metal oxide varistor having an alphain excess of 10 is shown in FIG. 1. The metal oxide varistor 1 is formedof a sintered ceramic metal oxide body 3. The body includes a firstmajor surface 5 and a second, opposed major surface 7. The majorsurfaces are separated by a thickness X. First and second electrodes 9and 11 are associated with the first and second major surfacesrespectively, so that they lie in ohmic contact therewith.

In placing the metal oxide varistor in use, when a potential differenceis placed across the electrodes 9 and 11, a current is conducted throughthe bulk of the metal oxide body 3. Since the distance between theelectrodes along the surface of the metal oxide varistor body is greaterthan through the bulk of the body, little, if any, current is conductedalong the surface of the body. For various voltage levels across theelectrodes the current follows equation 1. For a given crosssectionalarea of the metal oxide body measured normal to the direction of currentflow therethrough and for a given current level it has been observedthat the voltage across the electrodes is a function of the thickness X.

For many circuit applications where relatively high voltage levels aredesired at a given current conduction level this relationship isconvenient, as it is quite simple to choose a thickness value X to yieldthe desired voltage characteristic for the varistor. In circuitapplications where a relatively low voltage value is desired, however,for a given current conduction by the varistor, the value of X maybecome so small that it is quite difficult either to form or to handlethe metal oxide varistor body without damage. For example, forcomparatively low voltage applications a thickness for the metal oxidevaristor body of only 2 or 3 microns may be indicated. Further, whencomparatively low voltage characteristics are desired, the correctdimensions of the metal oxide body 3 can become quite important, as anerror in thickness of only a few microns might double or halve thedesired voltage characteristic.

It is an object of my invention to provide a metal oxide varistorconstruction in which the voltage characteristic is independent of thethickness of the metal oxide body. It is a more specific object of myinvention to provide a metal oxide varistor in which a rugged and easilyformed metal oxide body having no critical dimensions can be employedfor even the lowest voltage applications. It is still another object toprovide a metal oxide varistor according to my invention which isselfprotected from overloading. It is an additional object to provide ametal oxide varistor capable of clamping at multiple voltage levels.

In one aspect, my invention is directed to the combination comprised ofa substrate having first and second opposed major surfaces comprised ofa sintered ceramic metal oxide varistor body lying along at least thefirst major surface and having an alpha in excess of 10 in the currentdensity range of 10" to l0 amperes per square centimeter. First andsecond electrodes lie in ohmic contact with the first major surface andare laterally spaced to form a conduction gap therebetween along thefirst major surface having a minimum width less than the thickness ofthe substrate between the major surfaces.

My invention may be better understood by reference to the followingdetailed description considered in conjunction with the drawings, inwhich FIG. 1 is a schematic sectional view of the conven tional metaloxide varistor discussed above;

FIG. 2, 3, and 4 are schematic sectional views of separate embodimentsaccording to my invention;

FIG. 5 is a schematic circuit diagram;

FIGS. 6 and 7 are schematic sectional views of additional embodimentsaccording to my invention;

FIG. 8 is a schematic circuit diagram utilizing the embodiment of FIG.7;

FIG. 9 through 12 inclusive are plan views of additional embodimentsaccording to my invention; and

FIG. 13 is a schematic sectional view of a packaged unit incorporatingthe varistor of FIG. 3.

In FIG. 2 a varistor 20 is shown formed according to my invention. Thevaristor includes a metal oxide varistor body 21 having an alpha asdefined by equation 1 in excess of 10. The metal oxide varistor body maybe formed according to the teaching of the Canadian patent cited aboveor in any other known manner. The body is provided with a first majorsurface 22 and a second, opposed major surface 23. The second majorsurface, is shown to be parallel to the first major surface, but maytake any geometrical form convenient for the specific application towhich the varistor is to be placed. The thickness of the varistor bodymeasured normal to the major surfaces is not critical and may varywidely. The varistor body thickness is in most instances chosen so thatthe varistor body is rugged enough to avoid damage both in fabricationand handling. For example, the varistor body will normally exhibit athickness of at least 25 microns. In theory there is no limit to themaximum thickness of the varistor body, except that excessivethicknesses may unnecessarily add to the bulk and cost of the varistoras well as lengthening the thermal impedance path through the varistorbody.

Mounted on the first major surface is a first electrode 24 and a secondelectrode 25. The electrodes may be ohmically conductively associatedwith the major surface in any convenient conventional manner. Theelectrodes are laterally separated by a width Y, referred to as theconduction gap width. In the varistor 20 the conduction gap extendslinearly across the first major surface and is of uniform widththroughout. The conduction gap width determines the voltage level to beobserved across the electrodes for a given current conduction level.Accordingly, it is desirable in most instances to precisely control thiswidth. This can be accomplished by positioning the electrodes usingknown masking techniques to assure that they are accurately spaced or byinitially forming a single electrode and thereafter relieving anintermediate portion of the electrode in a controlled manner to leavethe first and second electrodes in spaced relation.

The conduction gap width Y may be of any desired value, depending uponthe voltage desired for a given level of current conduction. The lateralspacing of the electrodes of the varistor 20 is, however, particularlyadvantageous'when the conduction gap width Y is less than the thicknessof the varistor body, as would be the case in comparatively low voltageapplications. To illustrate this, it is merely necessary to observe thatif an electrode spacing'of 2 microns between electrodes is indicated toyield the desired current and voltage characteristic for a varistor, itwould be necessary to form the varistor body 3 with the thickness -Xbeing a value of only 2 microns. However, in my varistor 20 the varistorbody 21 can be formed of any convenient thickness. It is only theconduction gap width Y that must be controlled at 2 microns. Bycomparison to forming the varistor body itself .of this small thickness,like spacing of the electrodes is quite simple to accomplish employingtechniques well known to the art.

The operation of the varistor 20 differs from that of a conventionalvaristor as shown in FIG. 1. When a potential is impressed acrosselectrodes 24 and 25, the current that is conducted between theelectrodes is along or immediately beneath the surface of the varistorbody within the conduction gap Y. This is in direct contrast to theconventional varistor in which the current is more or less uniformlydistributed within the bulk of the varistor body. Of course, in thevaristor 20 there will be some fraction of the current that will becarried through the bulk of the varistor body beneath the surface of thebody, particularly as higher voltages are reached, but this should stillbe only a small proportion of the total current and may under mostcircumstances be considered negligible. Hence, while the varistor 2 willfollow equation 1 similarly as varistor 1, its internal conduction modeis quite dissimilar.

In addition to the varistor 20 I have also invented various alternativeembodiments differing in one or more functional and structural aspects.Except for the specific differing features noted and discussed, theremaining embodiments of my invention should be understood to employstructural characteristics identical to those of the varistor 20. 1

In FIG. 3 a varistor 30 is shown, which is a modified form of myinvention. A sintered ceramic metal oxide varistor body 31 is providedhaving a first major surface 32 and a second major surface 33.Electrodes 34 and 35, identical to electrodes 24 and 25, are associatedwith the first major surface and are separated by conduction gap widthY. A dielectric support 36 is associated with the second major surface.The dielectric support may be chosen from any one of a variety ofelectrically insulative, comparatively inert materials, such as, but notlimited to, known glass, ceramic, and polymeric insulators. Theadvantage of using the support 36 is that the thickness X3 of thevaristor body can new conveniently be reduced, since the ruggedness ofthe varistor body itself is supplemented to a considerable extent bythe'support. It is still a uniquely advantageous feature of my inventionthat the combined thickness of the varistor body and support, whichtogether form a common substrate, can be greater than the conduction gapY, although this is not absolutely essential to all applications of myinvention. It is recognized that in some circumstances, particularlywhen the support is a ceramic, it may be advantageous to form thevaristor body as a coating on the upper surface of the support. Thevaristor body and dielectric support can be bonded together to form aunitary substrate by conventional bonding techniques.

In FIG. 4 a varistor 40 is shown provided with a varistor body 41, whichmay be identical to 21, having a first major surface 42 and a second,opposed major surface 43. A first electrode 44 is ohmically conductivelyassociated with a portion of the first major surface. A second electrode45 is provided with a portion 45A ohmically conductively associated withthe first major surface and laterally spaced from the first electrode byconduction gap width Y. A remaining portion 45B of the second electrodeis associated with the second major surface, and an intermediate portion45C ohmically conductively connects the portions 45A and 45B of thesecond electrode. It is to be noted that the first and secondmajorsurfaces of the varistor body and,

hence, the first electrode and the portion 45B of the second electrodeare separated by a thickness X2, which exceeds the conduction gap widthY.

When the varistor 40 is called upon to conduct low current levels, itsoperation is identical to that of varistor 20. That is, current isconducted almost exclusively across conduction gap Y, and a relativestable low level voltage range (compared to that obtainable using aresistor) is maintained across the electrodes. Should, however, thevoltage level continue to rise across the first and second electrodes,as might occur in the case of a high power surge requiring currentconduction beyond the capacity of the conduction gap at the first majorsurface, the voltage across the electrodes can be stabilized again at asomewhat higher voltage level determined by the spacing X2 between thefirst electrode and the portion 45B of the second electrode. This willbecome more apparent when it is recognized that the conduction gap widthY, though lower in value than the thickness X2, relies for currentconduction upon a relatively restricted area of the varistor body lyingadjacent or immediately below the surface of the conduction gap, and forthis reason its current conducting capabilities are limited. By contrastthe somewhat more widely spaced first electrode and portion 458 of thesecond electrode are capable of conducting current therebetween throughthe bulk of the varistor body over a relatively extended area. In thisinstance it can be seen that the varistor 40 combines the very lowvoltage characteristics of the varistor while also incorporating as anadded feature the larger power handling capability of a conventionalvaristor, such as shown in FIG. 1, which also offers a second range ofvoltage stabilization.

Each of the varistors 20, 30, and 40 can be placed in an electricalcircuit to provide a shunt path around a high voltage degradable circuitunit, as :is illustrated in FIG. 5. The varistor is connected in thecircuit to selectively shunt current around the degradable unit inproportion to the voltage across the terminals 50 and 51. The currentthrough the varistor rises exponentially with any increase in voltageand hence serves to stabilize the voltage across the terminals.

In FIG. 6 a varistor 60 is illustrated. The varistor includes a varistorbody 61 having a first major surface 62 and a second major surface 63opposed thereto. Associated with the first major surface are first,second, and third electrodes 64, 65, and 66, respectively. Theelectrodes are each laterally spaced with the second electrode beinginterposed between the first and third electrodes. The first and secondelectrodes are separated by a conduction gap width Y1 and the second andthird electrodes are separated by a conduction gap width Y2, whichexceeds conduction gap width Y1 in value. The varistor 60 possesses allthe advantages of the varistor 20 plus the added advantage that thefirst and third electrodes can be simultaneously and independentlyreferenced to the second electrode. Further, by controlling theplacement of the second electrode 62 with respect to the first and thirdelectrodes, the resistance to current flow between the first and secondelectrodes can be related to the resistance to current flow between thesecond and third electrodes to provide any desired ratio of theseresistances. For certain applications the gap Width Y1 and Y2 may beequal in value.

In FIG. 7 a varistor 70 is illustrated which is provided with a varistorbody 71 that may be identical to varistor body 41. Adjacent first majorsurface 72 first and second electrodes 74 and 75 are located separatedby conduction gap width Y. A third electrode 76 is associated with thesecond major surface 73. The third electrode is separated from the firstand second electrodes by a thickness X2 of the varistor body. Thethickness X2 exceeds the gap width Y. Both the first and secondelectrodes can be referenced to the third electrode while at the sametime being referenced at a lower voltage range with respect to eachother.

A specific application for the varistor 70 is shown in FIG. 8. Circuitterminals 80 and 81 are shown. These terminals may be connected to aseries related electrical load and power source. The anode terminal 82and the cathode terminal 83 of an SCR 84 are shown connected to theterminals 80 and 81, respectively. Gate terminal 85 of the SCR isconnected to the cathode of a diode 86 and the anode of the diode isconnected to other conventional trigger circuitry 87 which is in turnelectrically connected to the terminals 80 and 82. The first electrode74 of the varistor is connected to the gate terminal 85. The secondelectrode 75 of the varistor is connected to the SCR anode terminal 82,and the third electrode 76 of the varistor is connected to the cathodeterminal 83 of the SCR.

It can be seen that in circuit operation the varistor acts as a shuntacross the SCR 84. Should a voltage surge develop across the SCR itwould be shunted through the varistor body between the second and thirdelectrodes and 76. At the same time the varistor 70 is also capable ofshunting a lower voltage that might develop across the diode 86 andcoventional trigger circuitry 87. This could occur, for example, if areverse voltage were applied to the SCR well within its voltage blockingcapability, but approaching the voltage blocking capability of the diode86. In this instance the diode is protected by the varistors voltageclamping ability through conduction gap width Y between the first andsecond electrodes. It is to be further noted that the portion of thevaristor having the highest power handling capability is used to protectthe power handling portion of the circuit, namely the SCR, while theportion of the varistor having a lower power handling capability, thefirst major surface associated conduction gap, protects the signalportion of the circuit. It is to be still further noted that excessivegate voltages are prevented by the varistor, since in this instanceconduction can occur through the varistor body between electrodes 74 and76. It is recognized that the varistor 60 could be substituted for thevaristor 70 in the circuit shown iwth first electrode 62 being connectedto gate terminal 85, second electrode 65 connected to anode terminal 82,and third electrode 66 connected to cathode terminal 83.

While I recognize that the limited current carrying capacity of myvaristors may be a disadvantage in certain applications requiringsubstantial power handling capabilities, their current handlingcapabilities may be enhanced by increasing the distance traversed by theconduction gap over the major surface so that it exceeds the maximumdimension of the major surface. In other words, the conduction gap neednot extend linearly across the major surface as described for simplicityin the foregoing embodiments.

A simple approach for increasing the distance traversed by theconduction gap on a major surface of a varistor according to myinvention is best appreciated by reference to FIG. 9. In this figure isshown a varistor 90 having a circular first eleectrode 91 and an annularsecond electrode 92 which is concentric with the circular electrode andwhichis uniformly spaced from the circular electrode by a conduction gapwidth Y. It may be readily observed that the distance traversed by theconduction gap exceeds the outer diameter of the annular electrode 92.In this way the current carrying area is increased over what would bepresent if two semicircular electrodes were employed in association withthe same underlying varistor body.

In FIG. 10 an approach for further increasing the area available forcurrent conduction is illustrated. A varistor is provided with a centralfirst electrode 101 having a plurality of regularly spaced fingers 102extending radially outwardly. An outer electrode 103 is provided with aplurality of radially inwardly spaced fingers 105 interdigitated withthe fingers 103. In this arrangement a variable spacing between theinner and outer electrodes is required if an equal amount of current isto be conducted throughout the conduction gap, as the differentcurvatures presented by the different portions of the fingers willproduce differing electrical fields if a uniform spacing is employed.Where unequal stresses can be tolerated on the fingers, it may be mostconvenient to provide a uniform spacing between the fingers or anapproximately uniform spacing.

In FIG. 11 a varistor 110 is illustrated which is provided with a firstelectrode 111 and a second electrode 112 associated in laterally spacedrelation to an underlying varistor body. The electrodes are formed sothat they are laterally separated by a minimum conduction gap width Y3and progressively diverge to a maximum conduction gap width Y4. Theeffect of varying the conduction gap width in this manner is to causethe varistor to present a somewhat lower alpha than should be presentbased upon the characteristics of the varistor body, per se. Thisapproach is particularly useful in using metal oxide varistorsincorporating varistor bodies having an alpha in excess of 10 in thecurrent den sity range of from 10 to 10 amperes per square centimeter toreplace previously utilized varistors, such as selenium and siliconcarbide varistors having alphas appreciably below 10.

In FIG. 12 a varistor 120 is illustrated having a first electrode 121and a second electrode 122. The two electrodes are laterally spaced on avaristor body in two discrete stepped increments. The left hand portionof each electrode is laterally spaced by a conduction gap width Y5 whichis less than the conduction gap width Y6 of the right hand portion ofeach electrode. It has been observed that with a constant direct currentbias placed across the electrodes of a metal oxide varistor a gradualincrease in the voltage level across the electrodes can occur as anaging function, particularly where the device is biased at near itspower handling capacity. In the varistor 120 the voltage between theelectrodes will initially be determined by the conduction gap width Y5.As the varistor ages in use it is possible that the voltage across thegap width Y5 may approach the voltage level in which the right handportion of the device becomes active. In this way an aging device isprotected against runaway voltages developing for a period of timepermitting replacement before uncontrolled voltage increase occurs.

As described above the varistors formed according to my teachings arefree of any protective packaging or external lead connections. In theform shown the varistors may be utilized in protected environmentswithout additional packaging. For example, the varistors could beincorporated in a hermetically sealed housing alone or in combinationwith other electrical components. For most applications it will bedesirable to attach terminal leads to the electrodes and to encapsulatethe varistors in a dielectric material to assure protection fromenvironmentally encountered substances altering their electricalcharacteristics.

To illustrate the packaging of a varistor formed according to myinvention, the varistor 30 shown in FIG. 3 is illustrated as thepackaged varistor 130 shown in FIG. 13. Elements of the varistor 130corresponding to those of the varistor 30 are assigned like referencecharacters and are not redescribed. Terminal leads 134 and 135 aresoldered or otherwise suitably attached in low impedance relation to theelectrodes 34 and 35, respectively. A substantially imperviousdielectric body 136, preferably formed of a dielectric glass of a typeconventionally employed in the passivation and/or packaging ofsemiconductor crystals, is shown overlying the conduction gap and theadjacent edges of the electrodes. Inasmuch as the conductioncharacteristics ofthe varistor are most appreciably influenced by theconduction properties present at or near its surface along theconduction gap, this is the area to which maximum protection should begiven. Note should be taken of the fact that this relationship isdirectly in contrast to that for the varistor l in which conductionoccurs through the bulk of the varistor body.

It is anticipated that for many applications the only protectivepackaging needed or desired for the varistor will be the dielectric bodycovering the conduction gap. For more general applications, however, itis normally desirable that an additional dielectric covering 137,

which may take the form of any conventional plastic or glasssemiconductor packaging composition, be used to cover the remainingexterior surfaces of the varistor body and, optionally, its electrodes.It is further anticipated that the packaging dielectric 137 may be usedalone with the dielectric body 136 being omitted. As shown, thedielectric package cooperates with the dielectric substrate 36 tocompletely cover the exterior surfaces of the varistor body. Where thevaristor is of a form lacking a dielectric substrate, it is appreciatedthat the package dielectric 137 may also completely envelop the varistorbody and, optionally, its attached electrodes.

While I have described my invention with reference to certain preferredembodiments, it is appreciated that numerous variations in form willreadily occur to those skilled in the art. For example, while I havedisclosed the varistor bodies to be of limited and regular lateralextent, it is appreciated that the lateral extent of the varistor bodybeyond the conduction gap width is not critical to its currentconduction capabilities. For this reason I contemplate that varistorsaccording to my invention may be formed with the varistor body extendinglaterally well beyond (or short of) the electrode outer edges, ifdesired, and utilizing lateral outline geometries of any convenientregular or irregular configuration. In the forms of my varistors withelectrodes attached to only one major surface it is appreciated that itis unnecessary to provide a second major surface of any regulargeometrical form or to have the second major surface parallel to thefirst major surface. While in most instances it will be most convenientto form the major surfaces so that they lie in a single'plane, it isanticipated that the major surfaces may, if desired, be curved or bent,so that they lie in more thanone plane. While I have shown a varistorfor purposes of illustration to be packaged as a lead mounted device,itis anticipated that the varistors formed according to my invention maybe attached to electricalterminals of various configurations. It isspecifically contemplated that the terminals attached may also serve todraw heat from the varistor body, as is well understood in thefabrication of electronic components. In this regard it is noted thatthe varistor body is itself a fairly good thermal conductor and willdissipate heat readily from the area immediately underlying theconduction gap. While a specific form of electrode interdigitation hasbeen shown for purposes of illustration, it is appreciated thatelectrode interdigitation is per se well known in the electroniccomponents arts and that many alternate forms of electrodeinterdigitation could be easily substituted.

Still other variations are contemplated. It is accordingly intended thatthe scope of my invention be determined by reference to the followingclaims.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. The combination comprising a substrate having first and secondopposed major surfaces comprised of a metal oxide varistor body lyingalong at least said first major surface and having an alpha in excess ofin the current density range of from 10" to 10 amperes per squarecentimeter and first and second electrodes lying in ohmic contact withsaid first major surface and laterally spaced to form a conduction gaptherebetween along said first major surface having a minimum width lessthan the thickness of said substrate between said major surfaces.

2. The combination comprising a substrate having first and secondopposed major surfaces;

said substrate consisting of a metal oxide varistor body;

said varistor body having an electrical resistance which varies as afunction of applied voltage in accordance with the formula I )alphawhere V is the voltage in volts applied to the body, I is the current inamperes through the body resulting from such voltage, and C and alphaare constants;

said body having an alpha in excess of 10 in the current density rangeof from 10 to 10 amperes per square centimeter;

first and second electrodes lying in ohmic contact with said first majorsurface and laterally spaced on said first major surface to form aconduction gap therebetween along said first major surface;

said conduction gap having a minimum width less than the thickness ofsaid varistor body measured normal to said first major surface.

3. The combination according to claim 2 wherein said varistor bodycomprises predominantly zinc oxide.

4. The combination according to claim 1 in which said substrate isadditionally comprised of a dielectric support associated with saidvaristor body.

5. The combination according to claim 1 in which a third electrode liesin ohmic contact with said first major surface, said third electrodelying in laterally spaced relation with said second electrode andlaterally separated from said first electrode by said second electrode,said second and third electrodes forming a conduction gap therebetweenalong said first major surface having a minimum width along said firstmajor surface exceeding the maximum width along said first major surfaceof the conduction gap between said first and second electrodes.

6. The combination according to claim 1 in which said substrate isformed entirely by said varistor body and additionally including a thirdelectrode lying in ohmic contact with said second major surface.

7. The combination according to claim 1 in which adjacent edges of saidfirst and second electrodes are substantially parallel.

8. The combination according to claim 1 including a metal oxide varistorbody having an alpha in excess of 10 within the current density range offrom 10' to 10 amperes per square centimeter and presenting at least onemajor surface,

first and second electrodes lying in ohmic contact with said body alongsaid major surface and laterally spaced relatively to form a conductiongap therebetween, and

dielectric means overlying said varistor body along the conduction gapand cooperating with adjacent edges of said electrodes to protect saidvaristor body against alteration of its electrical characteristics.

9. The combination according to claim 8 in which said dielectric meansand said electrodes together completely envelop said varistor body.

10. The combination according to claim 8 additionally including packaingmeans packaging with said dielectric means and said electrodes toenvelop said varistor body.

11. A varistor comprising a metal oxide varistor body having anelectrical resistance which varies as a function of applied voltage inaccordance with the formula I V/ )alpha where V is the voltage in voltsapplied to the body, I is the current through the body in amperesresulting from such voltage, and C and alpha are constants;

said body having an alpha in excess of 10 in the current density rangeof from 10' to 10 amperes per square centimeter; said body having atleast one major surface; first and second electrodes lying in ohmiccontact with said major surface and laterally spaced on said majorsurface to form a conduction gap therebetween along said major surface;said conduction gap having a minimum width less than the thickness ofsaid body measured normal to said major surface. 12. The combinationcomprising a metal oxide varistor body having an alpha in excess of 10in the current density range of from 10 to 10 amperes per squarecentimeter and presenting first and second opposed major surfaces, firstand second electrodes lying in ohmic contact with said first majorsurface and laterally spaced to form a conduction gap therebetween alongsaid first major surface having a minimum width less than the thicknessof said body between said major surfaces, and a third electrode lying inohmic contact with said second major surface. 13. A varistor accordingto claim 11 wherein said varistor body comprises predominantly zincoxide.

1. The combination comprising a substrate having first and secondopposed major surfaces comprised of a metal oxide varistor body lyingalong at least said first major surface and having an alpha in excess of10 in the current density range of from 10 3 to 102 amperes per squarecentimeter and first and second electrodes lying in ohmic contact withsaid first major surface and laterally spaced to form a conduction gaptherebetween along said first major surface having a minimum width lessthan the thickness of said substrate between said major surfaces.
 2. Thecombination comprising a substrate having first and second opposed majorsurfaces; said substrate consisting of a metal oxide varistor body; saidvaristor body having an electrical resistance which varies as a functionof applied voltage in accordance with the formula I (V/C)alpha where Vis the voltage in volts applied to the body, I is the current in amperesthrough the body resulting from such voltage, and C and alpha areconstants; said body having an alpha in excess of 10 in the currentdensity range of from 10 3 to 102 amperes per square centimeter; firstand second electrodes lying in ohmic contact with said first majorsurface and laterally spaced on said first major surface to form aconduction gap therebetween along said first major surface; saidconduction gap having a minimum width less than the thickness of saidvaristor body measured normal to said first major surface.
 3. Thecombination according to claim 2 wherein said varistor body comprisespredominantly zinc oxide.
 4. The combination according to claim 1 inwhich said substrate is additionally comprised of a dielectric supportassociated with said varistor body.
 5. The combination according toclaim 1 in which a third electrode lies in ohmic contact with said firstmajor surface, said third elecTrode lying in laterally spaced relationwith said second electrode and laterally separated from said firstelectrode by said second electrode, said second and third electrodesforming a conduction gap therebetween along said first major surfacehaving a minimum width along said first major surface exceeding themaximum width along said first major surface of the conduction gapbetween said first and second electrodes.
 6. The combination accordingto claim 1 in which said substrate is formed entirely by said varistorbody and additionally including a third electrode lying in ohmic contactwith said second major surface.
 7. The combination according to claim 1in which adjacent edges of said first and second electrodes aresubstantially parallel.
 8. The combination according to claim 1including a metal oxide varistor body having an alpha in excess of 10within the current density range of from 10 3 to 102 amperes per squarecentimeter and presenting at least one major surface, first and secondelectrodes lying in ohmic contact with said body along said majorsurface and laterally spaced relatively to form a conduction gaptherebetween, and dielectric means overlying said varistor body alongthe conduction gap and cooperating with adjacent edges of saidelectrodes to protect said varistor body against alteration of itselectrical characteristics.
 9. The combination according to claim 8 inwhich said dielectric means and said electrodes together completelyenvelop said varistor body.
 10. The combination according to claim 8additionally including packaging means cooperating with said dielectricmeans and said electrodes to envelop said varistor body.
 11. A varistorcomprising a metal oxide varistor body having an electrical resistancewhich varies as a function of applied voltage in accordance with theformula I (V/C)alpha where V is the voltage in volts applied to thebody, I is the current through the body in amperes resulting from suchvoltage, and C and alpha are constants; said body having an alpha inexcess of 10 in the current density range of from 10 3 to 102 amperesper square centimeter; said body having at least one major surface;first and second electrodes lying in ohmic contact with said majorsurface and laterally spaced on said major surface to form a conductiongap therebetween along said major surface; said conduction gap having aminimum width less than the thickness of said body measured normal tosaid major surface.
 12. The combination comprising a metal oxidevaristor body having an alpha in excess of 10 in the current densityrange of from 10 3 to 102 amperes per square centimeter and presentingfirst and second opposed major surfaces, first and second electrodeslying in ohmic contact with said first major surface and laterallyspaced to form a conduction gap therebetween along said first majorsurface having a minimum width less than the thickness of said bodybetween said major surfaces, and a third electrode lying in ohmiccontact with said second major surface. pg,24
 13. A varistor accordingto claim 11 wherein said varistor body comprises predominantly zincoxide.